There has been known a traffic control system for improving smooth running of vehicles at an intersection or the like. In the traffic control system, a roadside unit that wirelessly transmits roadside unit information is installed at an intersection. The roadside unit information includes traffic-light information representing a command (a stopping command or a running command) provided to the vehicles by a traffic light. Each of the vehicles can control its own running in response to the command provided by the traffic light by receiving the roadside unit information from the roadside unit.
In the traffic control system, a vehicle wirelessly transmits vehicle information including the running state and the like of the vehicle to other vehicles, and wirelessly receives vehicle information including the running state of other vehicles from these vehicles. Due to this configuration, the vehicle can notify other vehicles behind of the running state of the vehicle, and can control the running of the vehicle corresponding to the running state of other vehicles in front.
In the traffic control system, the roadside unit and vehicles transmit information to the same channel (the same frequency band). Therefore, if a plurality of vehicles simultaneously transmit information, an interference occurs. Accordingly, any vehicle cannot receive the information from the roadside unit. For example, when many vehicles are stopping, there is a high possibility of occurrence of an interference.
For example, a traffic light provides a stopping command at an intersection, and the vehicles stop. Therefore, the number of vehicles increases at the intersection, thereby increasing the possibility of a vehicle information interference. Further, at the intersection, the roadside unit information including the traffic-light information is transmitted. Because the traffic-light information is important, it is desired that the vehicle can receive the roadside unit information at a higher rate. However, if there are many vehicles that are stopping due to a stopping command of the traffic light, there is a lower probability that the vehicles can receive the roadside unit information.